This disclosure relates to a method for improving the probability of discrimination of crack-like features from the numerous magnetic flux leakage signals routinely detected by in-line pipeline inspection (ILI) tools such as circumferential magnetization flux leakage (MFL) technology. One problem routinely encountered in application of MFL methods is discrimination of cracks and crack-like features, such as due to stress-corrosion cracking (SCC) from pipe geometric features, such as manufacturing or mill defects. While methods of using MFL ILI technology exists (such as disclosed in Published U.S. Pat. Appln. No. 20070222436, discussed below), the reliability of the discrimination by known methods may be improved.
Detection of a crack using MFL ILI tools requires disruption of an induced magnetic field for detection. MFL detects a high population of geometric features in addition to cracks, which negatively affects reliability. Moreover, flux leakage is impacted by stress level.
Published U.S. Pat. Appln. No. 20070222436 discloses a method for detecting stress corrosion cracking of pipelines, comprising the steps of: identifying pipeline locations and pipeline conditions that are amenable to inspection by a magnetic flux inline tool and by a TFI tool; performing two inspections on the pipeline, one inspection performed using the magnetic flux inline (MFL) tool and an other inspection performed using the TFI tool; aligning signal features resulting from the two inspections; identifying TFI signals occurring above an un-specified threshold; identifying MFL signals for a section of pipeline corresponding to the identified TFI signals; for the identified TFI signals, determining whether the MFL signals are below a second threshold level; designating the sections of the pipeline corresponding to identified TFI signals above the unspecified threshold and below the un-specified second threshold as a potential corrosion feature; identifying TFI signals that exceed a defined metal loss percentage; measuring a width and length of the signal features, and if the width and length of the signal feature exceed threshold crack width and length values, designating as a potential corrosion feature section of pipeline corresponding to the identified TFI signals.
While the methods disclosed in '436 may be useful in certain inspections, the methods do not describe a methodology for characterization of detection of cracks as a function of ILI run parameters. The '436 application also does not identify how much pressure differential is required between successive ILI runs to enable discrimination of crack-like features, or how to determine a pressure for a second inspection which reliably detects cracks but is not disruptive of the business of the pipeline owner or operator.